Coupling devices are utilized in many applications for making and breaking connections in lines or flow paths which transmit-flowable materials. It has been found that one application where coupling devices are particularly useful is in systems for dispensing electrically conductive coating materials, such as water-based paints. In systems of this type, a "voltage block" or air gap must be maintained between the source of electrically conductive material and the electrostatically charged dispensing devices which discharge the coating material onto a substrate. Without this voltage block, personnel operating the equipment can be exposed to serious electrical shock if they were to come into contact with a charged supply of coating material, or, alternatively, if the supply is grounded, the electrostatics associated with the dispensing devices will not function.
Voltage block systems for dispensing electrically conductive coating material which employ coupling devices are shown, for example, in U.S. Pat. Nos. 4,313,475 and 5,078,168, both owned by the assignee of this invention, and the disclosures of which are incorporated by reference in their entireties herein. In these systems, electrically conductive coating material is first transmitted from a supply to a first vessel or a first pump which are electrically isolated from one or more dispensing devices. When filled with coating material, the first vessel or first pump is disconnected from the supply and then connected to a second vessel or a second pump, which, in turn, is connected to the dispenser(s). After the second tank or second pump is filled with coating material from the first vessel or first pump, the connection therebetween is broken allowing the second tank or second pump to supply coating material to the dispensers while the first vessel or first pump receives additional coating material from the source.
Coupling devices are very important to systems of the type described above in order to effectively make and break the connections between the first and second pumps or tanks, and the coating supply. It has been found that the coupling devices employed in U.S. Pat. No. 4,313,475, for example, are susceptible to leakage and/or drippage, and thus present clean-up problems. In addition, leakage of such connections can result in grounding of the system in which case the electrostatic coating dispensers cannot function. Further, such drippage can create an electrical shock hazard should a stream of dripping coating material contact an ungrounded object which is then touched by an operator.
The problem of drippage or leakage between mating elements of coupling devices employed in voltage block systems of the type described above has been addressed in U.S. Pat. No. 5,078,168 owned by the assignee of this invention. The '168 patent discloses a coupling device having mating male and female coupling elements which engage one another with a three-part seal to avoid leakage. The female coupling element is effective to "snuff back" or draw a vacuum at the outer end thereof which pulls in excess coating material present at the outer portions of the male and female coupling elements when they are decoupled. This creation of a suction or negative pressure at the outer end of the female coupling element substantially prevents drippage of coating material onto the floor or the surrounding apparatus, thus avoiding time consuming clean up and the potential problems of grounding of the coating dispensers and/or creating an electrical shock hazard.
While an improvement over previous coupling devices, the coupler disclosed in the '168 patent is potentially susceptible to problems of contamination of the coating material flowing therethrough after a color change or a cleaning operation has been performed. As disclosed in the '168 patent, when a coating operation is completed with a coating material of one color, the voltage block system and coupling device are cleaned with water or ocher solvent material in preparation for dispensing a coating material of different color. This cleaning procedure is necessary because in the course of a coating operation the coating material can collect on portions of the system, including the male and female coupling elements. It has been found that the configuration of the outer ends of the male and female coupling elements of the coupling device disclosed in the '168 patent can result in the retention or pooling of coating material thereon even after a cleaning operation is performed. As a result, when the system is operated with a new coating material of different color, the previous coating material or cleaning solvent which was retained on the coupling elements can come into contact with the new coating material thus creating problems of contamination. Additionally, the seal between the coupling elements of the '168 patent relies, at least in part, on o-rings located on each of the coupling elements. These o-rings become covered with coating material which dries causing the o-rings, in some instances, to be pulled away from their seats upon engagement and disengagement of the coupling elements.